i was hoping you might chime in on this seeing how this clear is your go-to. we don't know what the OP is starting with in regards to panel prep, but...
at this point i'd be hesitant to add hardener. i'd also go with the least amount of thinner required to get a smooth finish. 5-10% maybe, preferably...
i do that regularly. it takes me ages to do an entire vehicle so i go in sections just like you. i like to epoxy over the metalwork and apply filler...